If you plant to make an unofficial site event and organise/oversee it fully then you don't need to ask permission really. Unless you require cooperation or rewards from the site artists/admins/mods.
If you want to coordinate with the rest of the forum, mods and golds, then you're free to relay your ideas to others who would want to help you.
Gold forum is originally designed for discussion on contests and that's where a lot of the planning happens.
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